Solution deposition of planar films of the hybrid perovskite (HP) methylammonium (MA) lead iodochloride (MAPbI3-xClx) often results in very low surface coverage, small grain size, and high density of defects, particularly for the pure iodide HP. These decrease the optoelectronic quality of MAPbI3 (minority lifetimes all less than 10 ns) and creates pinholes that may result in shunt pathways that severely degrade the efficiency of photovoltaic devices. The poor morphology is usually attributed to the formation of large disconnected grains of PbI2 that nucleate first and set the morphology of the final HP layer. As a result, many use PbCl2 as a lead source. The PbCl2 is less soluble, forms smaller grains, and promotes more continuous HP films. Here, we show a highly reproducible deposition method for pure iodide MAPbI3 that yields continuous films with large grain sizes and minority carrier lifetimes greater than 200 ns. The method consists of thermal evaporation of PbI2 and a post-deposition Vapor-Equilibrated Re-Growth (VERG) step at 110 °C in a closed vessel.
Zinc cadmium sulfide (ZnxCd1-xS) thin films grown through chemical bath deposition are used in chalcopyrite solar cells as the buffer layer between the n-type zinc oxide and the p-type light absorbing chalcopyrite film. To optimize energetic band alignment and optical absorption, advanced solar cell architectures require the ability to manipulate x as a function of distance from the absorber-ZnCdS interface. Herein, we investigate the fundamental factors that govern the evolution of the composition as a function of depth in the film. By changing the initial concentrations of Zn and Cd salts in the bath, the entire range of overall compositions ranging from primarily cubic ZnS to primarily hexagonal CdS could be deposited. However, films are inhomogeneous and x varies significantly as function of distance from the film-substrate interface. Films with high overall Zn concentration (x > 0.5) exhibit a Cd-rich layer near the film-substrate interface because Cd is more reactive than Zn. This layer is typically beneath a nearly pure ZnS film that forms after the Cd-rich layers are deposited and Cd is depleted in the bath. In films with high overall Cd concentration (x < 0.5) the Zn concentration rises towards the film's surface. Fortunately, these gradients are favorable for solar cells based on low band gap chalcopyrite films.
Cu(2)ZnSnS(4) (CZTS) nanocrystals sterically stabilized with oleic acid and oleylamine ligands and dispersed in nonpolar organic liquids have been extracted into, and electrostatically stabilized in, polar liquids by covering their surfaces with S(2-).
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